The present invention relates to the technological field of vehicle posture control apparatuses that avoid or suppress understeering tendencies (drifting out) and oversteering tendencies (spinning) by controlling the posture of a vehicle during cornering.
Conventionally, various vehicle posture control apparatuses are known, in which, such as in JP H06-183288A or JP H07-223520A, a target yaw rate is set based on the steering wheel angle and the vehicle speed, the actual yaw rate of the vehicle is detected with a yaw rate sensor, and when the deviation between the detected actual yaw rate and the target yaw rate has at least a predetermined value, an understeer control suppressing an understeering tendency of the vehicle intervenes or an oversteer control suppressing an oversteering tendency of the vehicle intervenes.
More specifically, in such a posture control apparatus, understeer control intervenes when the target yaw rate is larger than a value obtained by adding a predetermined threshold to the actual yaw rate, whereas oversteer control intervenes when the actual yaw rate is larger than a value obtained by adding a predetermined threshold to the target yaw rate.
In this conventional posture control apparatus, too much oversteer control tends to intervene when the threshold is small. When the oversteer control intervenes too early, there is the problem that this invites difficulties in turning the vehicle due to excessive control, and also the strength of the operation when intervention of control is unnecessary is a problem.
In order to rectify this, the threshold determining the intervention of the oversteer control should be increased, so that an early intervention of the oversteer control can be inhibited. However in that case, the control will intervene only when the oversteering tendency of the vehicle is already too large. Moreover, when the control finally intervenes, a strong control intervenes abruptly. Therefore, even though it is possible to ensure the stability of the vehicle, in a situation in which the oversteering tendency keeps increasing until the oversteer control intervenes, the driver may feel that the controllability worsens, leading to a sense of instability.
In view of these problems, it is an object of the present invention to ensure a high vehicle stability and to enhance the sense of stability and the controllability felt by the driver by improving the oversteer control.
In order to achieve these objects, according to a first aspect of the present invention, in addition to a first oversteer control for suppressing an oversteering tendency, a second oversteer control is provided, whose control amount is lower than that of the first oversteer control.
More specifically, the subject matter of the first aspect of the present invention is a vehicle posture control apparatus provided with a control means for controlling posture of the vehicle in yawing direction by independently controlling the brakes of the vehicle""s wheels.
A special feature of the invention is that the control means carries out intervention of a first oversteer control suppressing the oversteering tendency when the oversteering tendency of the vehicle is stronger than a first preset reference value, and the control means carries out intervention of a second oversteer control, in which the control amount is lower than in the first oversteer control, when the oversteering tendency of the vehicle is not stronger than the first preset reference value, but stronger than a second preset reference value.
Thus, when the oversteering tendency of the vehicle is stronger than a first preset reference value, that is, when the oversteering tendency is relatively strong, the first oversteer control intervenes to suppress this oversteering tendency. This sufficiently ensures the stability of the vehicle.
Then, when the oversteering tendency of the vehicle is not stronger than the first preset reference value but stronger than the second preset reference value, or in other words, when there is a relatively weak oversteering tendency, intervention of the second oversteer control, whose control amount is lower than that of the first oversteer control, is carried out. Thus, by early intervention of the second oversteer control, this weak oversteering tendency is suppressed, and also growth of the oversteering tendency (the oversteering tendency becoming stronger) is suppressed. Thus, the stability of the vehicle is increased even more, improving the sense of stability felt by the driver, while strong oversteering tendencies are suppressed, and the ease of control felt by the driver is improved.
The second oversteer control is a weak control with a lower control amount. Therefore, the second oversteer control does not lead to excessive control even when it intervenes early, and unnecessary operation can be prevented from becoming too strong. As a result, an awkward feeling of the driver is prevented.
Furthermore, when for example the oversteering tendency increases even though the second oversteer control has intervened, and the oversteering tendency becomes stronger than the first preset reference value, then the first oversteer control intervenes, replacing the second oversteer control. Thus, the first oversteer control, which has a strong control amount, does not intervene abruptly. Also, the awkward feeling of the driver is largely rectified by continuously moving from the second oversteer control, which is a weak control, to the first oversteer control, which is a strong control. Also, the second oversteer control intervenes previously to intervention of the first oversteer control, so that the play of the brake system is eliminated (for example, leading to a state in which the brake pads adhere to the disk rotor). Therefore, the responsiveness of the first oversteer control is improved. Furthermore, if the first oversteer control intervenes in continuation to the intervention of the second oversteer control, the effect is substantially the same as when lowering the threshold of the oversteer control, so that an even higher stability of the vehicle can be ensured.
Consequently, providing the second understeer control separately from the first understeer control ensures a high stability of the vehicle while improving the sense of stability and the controllability felt by the driver.
It is preferable that the second oversteer control supplies, by open control, a brake pressure whose upper limit is a predetermined brake pressure that is lower than the maximum brake pressure that can be supplied in the first oversteer control.
That is to say, the responsiveness of the control is improved by supplying the brake pressure by open control. At the same time, a suppression of an oversteering tendency with a control amount that is lower than in the first oversteer control is achieved by setting a predetermined brake pressure that is lower than the maximum brake pressure that can be supplied in the first oversteer control. This way, a relatively weak oversteering tendency can be suitably adjusted without the driver noticing the intervention of a control.
It is preferable that the second oversteer control supplies brake pressure in accordance with a deviation between a target yaw rate that has been set and the actual yaw rate.
That is to say, by supplying the brake force by feedback control in accordance with a deviation between a target yaw rate that has been set and the actual yaw rate, there is no excessive suppression control of the oversteering, and an optimal control is achieved. It should be noted that the upper limit of the brake pressure can also be set to a predetermined brake pressure that is lower than the brake pressure during the first oversteer control. In this case, it is easy to achieve a suppression control of oversteering, in which the control amount is lower than in the first oversteer control.
It is preferable that the control means prohibits intervention of the second oversteer control when the vehicle has an understeering tendency.
That is to say, if the second oversteer control would intervene when the vehicle is in an oversteering tendency while also being in an understeering tendency, for example when the vehicle drifts out while spinning, then the understeering tendency would be promoted. Therefore, when the vehicle has an understeering tendency, the intervention of the second oversteering tendency should be prohibited.
In a second aspect of the present invention, two oversteer controls, namely a second and a third oversteer control, whose control amounts are lower than that of the first oversteer control, are provided in addition to the first oversteer control for suppressing oversteering tendencies.
More specifically, the subject matter of the second aspect of the present invention is a vehicle posture control apparatus provided with a control means for controlling posture of the vehicle in yawing direction by independently controlling brakes of the vehicle""s wheels.
A special feature of the invention is that the control means carries out intervention of a first oversteer control suppressing an oversteering tendency when the oversteering tendency of the vehicle is stronger than a first preset reference value; the control means carries out intervention of a second oversteer control, in which the control amount is lower than in the first oversteer control, when the oversteering tendency of the vehicle is not stronger than the first preset reference value, but stronger than a second preset reference value; and the control means carries out intervention of a third oversteer control, in which the control amount is lower than in the first oversteer control, when the oversteering tendency of the vehicle is not stronger than the second preset reference value, but stronger than a third preset reference value.
Thus, when the oversteering tendency of the vehicle is stronger than a first preset reference value, the first oversteer control intervenes to suppress this oversteering tendency. This suppresses relatively strong oversteering tendencies of the vehicle.
Then, when the oversteering tendency of the vehicle is not stronger than the first preset reference value but stronger than the second preset reference value (when there is a relatively weak oversteering tendency), intervention of the second oversteer control, whose control amount is lower than that of the first oversteer control, is carried out.
Moreover, when the oversteering tendency of the vehicle is not stronger than the second preset reference value but stronger than the third preset reference value (when there is an even weaker oversteering tendency), intervention of the third oversteer control, whose control amount is lower than that of the first oversteer control, is carried out.
Thus, when the vehicle has a relatively weak oversteering tendency, the second oversteer control intervenes early, suppressing this oversteering tendency and suppressing an increase of the oversteering. As a result, the sense of stability as well as the ease of control felt by the driver are improved. The second oversteer control intervening here is a weak control whose control amount is lower than that of the first oversteer control. Therefore, the control does not become excessive, and an unnecessary operation can be prevented from becoming strong.
Furthermore, when the vehicle is in an even weaker oversteering tendency, the third oversteer control intervenes early on. This suppresses the oversteering tendency and suppresses an increase of the oversteering. Thus, the sense of stability as well as the controllability felt by the driver are improved even more. Also the third oversteer control is a weak control whose control amount is lower than that of the first oversteer control. Therefore, the control does not become excessive, and an unnecessary operation can be prevented from becoming strong.
Furthermore, when the oversteering increases even though the third oversteer control has intervened, then the second oversteer control intervenes, replacing the third oversteer control. And when the oversteering increases even more even though the second oversteer control has intervened, then the first oversteer control intervenes, replacing the second oversteer control. This way, the control moves continuously from the third and the second oversteer control, which are weak controls, to the first oversteer control, which is a strong control. As a result, the awkward feeling of the driver is largely rectified. Furthermore, if the first oversteer control intervenes in continuation to the intervention of the third and the second oversteer control, the effect is substantially the same as when lowering the control threshold, so that an even higher stability of the vehicle can be ensured.
Consequently, by providing the second and the second and the third oversteer control separately from the first oversteer control, the sense of stability and the controllability felt by the driver are improved even more, while ensuring a high stability of the vehicle.
It is preferable that a supply ratio of brake pressure during the third oversteer control (amount of brake pressure supplied per unit time) is set to be lower than a supply ratio of brake pressure during the second oversteer control.
This prevents the control from becoming excessive when the third oversteer control intervenes during an extremely weak oversteering tendency. As a result, a suitable suppression control of oversteering tendencies can be achieved that is hardly noticed by the driver.
It is preferable that the second oversteer control supplies, by open control, a brake pressure whose upper limit is a predetermined brake pressure that is lower than the maximum brake pressure that can be supplied in the first oversteer control.
Thus, as explained above, when the second oversteer control intervenes during a relatively weak oversteering tendency, this weak oversteering tendency is suppressed without imparting an awkward feeling on the driver. Also, when the intervention of the first oversteer control succeeds the intervention of the second oversteer control, then the transition will be smooth.
It is preferable that the third oversteer control supplies brake pressure in accordance with a deviation between a target yaw rate that has been set and the actual yaw rate.
Thus, the third oversteer control for suppressing extremely weak oversteering tendencies is suitably performed without becoming excessive.
In addition, it is preferable that an upper limit of the brake pressure in the second and third oversteer control is set to 10 to 25 bar.
By setting the upper limit of the brake pressure in this range, suitable posture control of the vehicle is achieved, even though the driver will hardly notice it. It should be noticed that in order to achieve both the effect of controlling the posture of the vehicle and preventing an awkward feeling due to the driver noticing the intervention of control, it is most preferable that the upper limit of the braking pressure is 15 bar.
Also in the second aspect of the present invention, it is preferable that the control means prohibits intervention of the second oversteer control when the vehicle has an understeering tendency.
It is further preferable that the control means prohibits intervention of the third oversteer control when the vehicle has an understeering tendency.
As mentioned above, if the second or third oversteer control would intervene when the vehicle is in an oversteering tendency while also being in an understeering tendency, then the understeering tendency would be promoted. Therefore, when the vehicle has an understeering tendency, the intervention of the second or third oversteering tendency should be prohibited.